Technical Field
The present invention relates to a binder intended for a positive electrode of a lithium ion secondary battery and having excellent high-rate discharge characteristics, a positive electrode of a lithium ion secondary battery containing the binder, and a lithium ion secondary battery using the positive electrode.
Description of Related Art
In recent years, with widespread use of portable electronic devices such as laptop computers, smartphones, portable game machines, and PDAs, the need for reducing the size of second batteries used as power sources and increasing the density of energy has been growing in order to reduce the weight of the above-described devices and achieve the use of the above-described devices for a longer period of time.
Particularly in recent years, the use of secondary batteries as power sources for vehicles such as electric bicycles, electric motorcycles, and electric automobiles has been widespread. Batteries having a high density of energy and being capable of operating in a wide temperature range have been demanded as the secondary batteries used as the power sources for vehicles.
Conventionally, e.g., nickel-cadmium batteries and nickel-hydrogen batteries have been mainly used as secondary batteries. However, the use of lithium ion secondary batteries tends to increase due to the need for size reduction and a higher density of energy as described above.
Typically in a lithium ion secondary battery, lithium cobalt oxide (LiCoO2) is used as a positive electrode, a carbon electrode is used as a negative electrode, and a non-aqueous electrolytic solution formed in such a manner that lithium ions are dissolved in an organic solvent such as propylene carbonate is used as an electrolyte. Transition metal oxides containing lithium ions, such as lithium nickel oxide (LiNiO2) or spinel type lithium manganite (LiMn2O4), have been known as other types of positive electrode active material.
In these positive electrode active materials, the capacity and stability of the positive electrode active material are determined by reversible insertion/removal reaction of the lithium ions. The capacity of the positive electrode active material increases with increasing the amount of Li removed from the positive electrode active material. More removed Li results in a higher charge voltage.
However, due to an increase in the amount of Li removed from the positive electrode active material, breakdown of the crystal structure of the positive electrode active material and oxidative decomposition of a binder and an organic electrolyte due to an increase in the charge voltage may occur. As a result, there is a concern that battery characteristics such as high-rate discharge characteristics and cycle characteristics are degraded.
In order to improve the high-rate discharge characteristics, various proposals have been made to improve the positive electrode active material, a negative electrode active material, the electrolyte, and the electrolytic solution, but there is the limited number of proposals on the binder (see, e.g., Patent Documents 1, 2, and 3).
However, the binders disclosed in these patent documents are used in the form of a latex, an emulsion, or a liquid solution using an organic solvent, and there is a problem of increasing an environmental load due to the use of the organic solvent.
Moreover, in order to improve the cycle characteristics, the method of covering the positive electrode active material with oxide such as Al2O3, ZrO2, TiO2, SiO2, or AlPO4 has been proposed (see, e.g., Patent Document 4).
However, since these oxides are insulators, there is a problem that the conduction path of the lithium ions and the electron transfer path are blocked particularly in rapid charging/discharging to cause an increase in an electrode reaction resistance, resulting in a decrease in a battery capacity.
The electrodes of the lithium ion secondary battery are formed in such a manner that the active material, the binder, and a conductive assistant are applied onto a current collector and then are dried.
For example, the positive electrode is formed in such a manner that a slurry in which LiCoO2 as the active material, polyvinylidene fluoride (PVdF) as the binder, and carbon black as the conductive assistant disperse is applied onto an aluminum foil current collector and then is dried. However, since the PVdF does not dissolve or disperse in water, N-methyl-2-pyrrolidone (NMP) is required as the organic solvent, and therefore, there is a problem on the environmental load. In addition, the PVdF swells in the electrolytic solution under a high-temperature environment of equal to or higher than 50° C., resulting in weakening of binding force and an increase in an electrode resistance. As a result, the positive electrode lacks high-temperature durability.
On the other hand, the negative electrode is formed in such a manner that a slurry in which graphite as the active material, carboxymethylcellulose (CMC) and styrene-butadiene rubber (SBR) as the binders, and carbon black as the conductive assistant disperse is applied onto a copper foil current collector and then is dried. The CMC and the SBR dissolve or disperse in water, resulting in a less environmental load with a low cost.
In addition to the CMC and the SBR, a lithium secondary battery binder composition containing a crosslinked compound of polyacrylic acid subjected to exchange for alkali cations and polyvinyl alcohol has been proposed as an aqueous binder (see, e.g., Patent Document 5).
In Patent Document 5, in the case where the crosslinked compound of polyacrylic acid subjected to exchange for alkali cations and polyvinyl alcohol is used for the negative electrode as the lithium secondary battery binder, there is an advantage in improvement of the life characteristics of the electrode. However, the example where such a binder is used for the positive electrode has not been disclosed, and there is no description on the high-rate discharge characteristics remaining the issue unique to the lithium secondary battery.
Unlike the negative electrode, the reasons why it is difficult to use the aqueous binder for the positive electrode are, for example, as follows:                (1) in charging, oxidative decomposition of the aqueous binder occurs;        (2) it is difficult to uniformly disperse the slurry;        (3) if an attempt is made to improve the capacity of the positive electrode by an increase in the thickness thereof, cracking occurs at the electrode due to cohesive stress caused by drying; and        (4) the positive electrode active material and water contact and react with each other, and therefore, lithium of the positive electrode active material dissolves out to cause cracking of the positive electrode and to decrease the capacity of the positive electrode.        
Since a sufficient conduction path cannot be ensured in the electrode due to cracking in the electrode, there is a concern that the high-rate discharge characteristics as the battery characteristics are degraded and that a decrease in the capacity of the positive electrode and degradation of the cycle characteristics occur due to dissolving of lithium of the positive electrode active material.
It is often the case that active materials in a battery material system do not exhibit favorable battery characteristics merely by new combination of existing materials, resulting in no predictability. For such a reason, in evaluation of the battery material system, it is required even for the existing materials that the battery material system is evaluated as a battery and that the benefit of the battery material system is proved based on evaluation results. In other words, even when the material itself is known, if no evaluation is made on such a material as a battery, such a material is regarded as an unknown material in the battery material system. Moreover, if a battery does not operate as a whole, such a battery is useless. Thus, even if any useful active materials are used, the compatibility with a binder, a conductive assistant, and a current collector should be fully taken into consideration, and an antipole, an electrolytic solution, etc. are also important.